This invention relates to arrangements and methods for preventing distortion of integrated opto-electronic devices.
Complex opto-electric devices for example micro-electro mechanical systems (MEMS) devices are being introduced into communications networks to provide a number of functions including switching and light source applications. These devices are typically formed on a substrate formed from a ceramic material or from a single crystal semiconductor material. As the devices become more complex, larger area substrates are required.
A characteristic feature of these devices is the critical alignment that must be maintained between the optical components of the device in order to minimise loss of optical signals between those components.
A typical device of this type is the MEMs crosspoint switch. This device comprises an array of individually actuable mirrors formed in a silicon substrate and provides a non-blocking crossbar architecture in which switching is effected between a plurality of optical inputs and outputs. Devices of this type have the potential to become key components in the rapid expansion of the Optical Internet. However, the introduction of these devices is currently constrained by their extreme sensitivity to distortion arising from mechanical stress caused e.g. by thermal mismatch in the package assembly within which the device is housed. This extreme sensitivity to distortion is a particular problem for the larger packaged devices now being developed.
Optical crosspoint switches that are currently being proposed have a typical substrate size of about 60 mm square. These devices operate by switching of optical beams having a typical beam waist of about 240 microns and, to prevent degradation of performance, it is necessary to maintain accurate alignment of the beam. It has been found for example that, over a distance of 60 mm, it is necessary to ensure that the beam does not deflect from its true path by an angle greater than 0.5 milliradian. Beam deflections of this order can easily be caused by thermal mismatch and/or mechanical stress of the package in which the MEMs device is mounted. It will further be appreciated that as devices increase in size and complexity, there will be a corresponding increase in the problem of distortion.
A potential solution to the problem of distortion is to mount the MEMs device on a stable surface within an enclosed housing provided with a thermostatically controlled temperature. However, such an assembly is relatively costly. Further, such a structure occupies a significant physical volume, which can be a disadvantage where space is at a premium. In addition, this solution does not address the problem of long term ageing effects, which may need to be considered when a high reliability working life of many years is envisaged.
An object of the invention is to minimise or to overcome the above disadvantage.
A further object of the invention is to provide an improved device construction.
According to a first aspect of the invention, there is provided an optical device disposed on a first major surface of a substrate, the device having sensing means associated with the device for detecting distortion of the substrate, and feedback control means responsive to the sensing means for applying a force to the substrate so as to compensate for the effect of such distortion.
According to another aspect of the invention there is provided an optical device disposed on a first major surface of a substrate, the device having input and output optical signal paths, sensing means associated with a said output optical signal path and arranged to detect a reduction of a signal on that path corresponding to a distortion of the substrate, means for applying a force to the substrate, and feedback control means responsive to the sensing means for controlling said force applying means so as to negate said distortion of the substrate.
According to another aspect of the invention there is provided an optical micro-electro-mechanical system (MEMS) device disposed on a first major surface of a substrate, the device having a first set of input optical waveguides, a second set of output optical waveguides, an array of switch elements disposed on said first major surface and each arranged to couple selectively optical signals between a respective input optical waveguide and output optical waveguide, signal sensing means associated with one or more of said output optical waveguides and arranged to detect a distortion of the substrate as a reduction in amplitude of a signal carried on said one or more output optical waveguides, means for applying a force to the substrate, and feedback control means responsive to the sensing means for controlling said force applying means so as to negate said distortion of the substrate.
According to another aspect of the invention there is provided an optical device disposed on a first major surface of a laminar substrate, the device having sensing means associated with the device for detecting distortion of the substrate, and means for applying a controlled force to the substrate so as to compensate for the effect of such distortion.
According to another aspect of the invention, there is provided a method of distortion compensation of an optical device disposed on a laminar semiconductor substrate, the method comprising, detecting distortion of the substrate by measurement of an optical parameter of said optical device, and, in response to said distortion detection, applying a force to the substrate so as to negate such distortion.
Advantageously, the actuator comprises a layer of piezo-electric material applied to the substrate lower surface and to which a controlled voltage is applied to impart a force to that surface so as to negate a distortion hereof. Preferably, the piezo-electric material is pre-compressed. Typically, the distortion sensitive device comprises a MEMS device such as a crosspoint switch or a laser package.
The PZT material may be provided in sheet form and bonded to the substrate by a heat curing adhesive.
In a preferred embodiment, an oscillatory signal is superimposed on an electrical control signal applied to the layer of a piezo-electric material. Phase detection of a corresponding oscillatory signal in the measured optical parameter provides a feedback for increasing or decreasing the magnitude of the control signal.
It will be understood that the above description of a preferred embodiment is given by way of example only and that various modifications may be made by those skilled in the art without departing from the spirit and scope of the invention.